Dewatering clayey and silty soil



Sepf- 27, 1966 R. E. LANDAU 3,274,782

DEWATERING CLAYEY AND SILTY SOIL Filed July 5, 1965 2 Sheets-Sheet 1RICHARD E. LANDAU BY ma@ e 1 f 1,1/2

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Sept 27 1965 R. E. LANDAU 3,274,782

DEWATERING CLAYEY AND SILTY SOIL Filed July 5, 1963 2 Sheets-Sheet 2INVENTOR. RICHARD E. LANDAU BY 474 w Y f AT1-op ys.

United States Patent O 3,274,782 DEWATERING CLAYEY AND SILTY SOILRichard E. Landau, Middle Village, N.Y. (717 Cornwell Ave., WestHempstead, N.Y. 11552) Filed July 5, 1963, Ser. No. 293,163 6 Claims.(Cl. 61-11) This invention relates to a method for removing water orother liquid from clayey or silty soils which will not compress to anysubstantial degree by such removal, and is a contnuation-in-part ofapplication No. 746,099 led July 2, 1958, now Patent No. 3,096,622.

In the said Patent 3,096,622, a method for consolidating compressiblesoils by removing water and other liquids is described. This applicationis a continuation-in-part of said application and relates to a method ofdraining or removing water or other liquid from la clay or silty earthformation, wherein the earth formation does not compress to anysubstantial degree. In the usual case, soils of the type described willconsolidate in a vertical direction when a force is applied. However, insome instances the force applied does not exceed the intergranularpressure of the soil and consequently, consolidation does not occur toany appreciable degree. For example, an earth formation may have beenpreconsolidated such that subsequent loading will not produce a verticalforce in excess of the intergranular pressure of the preconsolidatedsoil. Nevertheless, it is often desirable to dewater a particular volumeof earth to permit construction below the natural water level withoutthe need for employing underwater equipment. The method of thisinvention provides a relatively rapid and economical process fordewatering clayey or silty soils.

Soils of the type described generally contain la substantial quantity ofwater (pore water) dispersed within the structural matrix. Such water ismost generally associated with the level of water in the soil, commonlyreferred toas the water table.

The rate ,at which the dewatering will occur is related to the abilityof soil matrix to permit the passage of pore water. The morefine-grained the soil the lower the rate of pore wate-r movement for agiven pressure differential, or in other words, the lower thepermeability of the soil.

Most clayey or silty soils are alluvial deposits, which were laid downin horizontal layers in the geologic past. Such deposits, generally,have a higher permeability in a horizontal direction along the beddingplane than in the vertical direction across the bedding planes. Thiscondition is particularly pronounced in varved deposits which consist ofalternating layers of ine and course grained materials. Varved depositswere generally formed by the stagnation of the continental glaciers, andare often found in rivers and lakes.

Clayey or silty soils generally have a greater horizontal than verticalpermeability. To take advantage of this phenomenon the constructionindustry has resorted to the use of vertical drains, such as,wellpoints, `and sand drains. Wellpoints are usually lengths of tubingdriven into the soil within the area to be drained; the tubing beingporous along the length at the lower end thereof. The tubing isconnected to a suction source such as .a pump which draws thesurrounding soil water into and through the tubing to the surface. Sanddrains are columns of sand constructed in the soil and generally used toexpedite flow of water and liquid under pressure, which may be inducedor otherwise exist, in the soil. These types of drains, which generallyare installed vertically but may in suitable cases be installed at anangle or even horizontally, are devices in the earth that serve as lowpressure points to which the pore water may ow and through which thepore watermay leave the soil forma- 3,274,782 Patented Sept. 27, 1966tion when a suitable pressure differential exists to cause ow. To createthe low pressure zone, a wellpoint is disposed in the soil at thedesired level thereof. The wellpoint when in operation creates a lowpressure zone by operation of a suction pump connected thereto whichaccelerates the ilow of water from the formation into the drain. Thewellpoint is so constructed that formation liquid precolating into thistype of drain will flow to the wellpoint, after which it is pumped tothe surface.

Often sand drains and Wellpoints are used together especially in clayand silt soils where the permeability is so low that a large contactarea between the drain and the soil is des-irable to achieve betterdrainage whereby the sand drains complement the drainage available fromthe wellpoint. In such an arrangement the sand drain is generallycircular in cross-section, with the wellpoint loc-ated to a suitabledepth within the sand column. lf desired a vacuum may be eifected in thesand drain by plugging the upper portion of the said column to preventthe entry of air and pumping from -a wellpoint disposed to a suitabledepth within said sand column.

In the case of sand drains, the hole is bacldilled with a material ofrelatively high permeability, such as coarse sand; other materials andother means are sometimes used to maintain the void and permit the freepassage of fluids.

Heretofore, it has been the practice to construct drain holes forinserting Wellpoints or defining sand drains by ramming or driving ahollow casing into the formation, and where applicable, lling the casingwith porous or other desired devices Aand material which in turn illsthe void formed by the displaced soil. Because of the nature of thesoils involved, ramming substantially decreases the horizontalpermeability of the soil and -results in a slower dewateriing rate thanwould otherwise be possible. This decrease in permeability is due to thecombined effect of smearing and remolding of the face of the voidcontiguous to the casing, as well as due to the disturbance of the soilin the vicinity of the hole by displacement yand energy input to thesoil by the driving of the casing.

I have discovered a method for constructing drain holes in 'an earthformation which avoids the disadvantages of the prior art. Generallystated, my invention comprises helically cutting into the earthformation to the desired depth, removing the section of soil traversedby cutting, positioning a wellpoint at the desired level in the cavityformed by removing the earth, and where the wellpoint itself does notsupport the hole suflciently, fill-ing the cavity with a porous mediumto -a suitable depth to permit the desired drainage. Where ya wellpointdrain is inserted into a sand drain column, a substantially airtightcover can be placed over or within the cavity above the porous wellpointthus permitting the formation of a low pressure vacuum zone uponactuating the wellpoint by pumping.

By cutting into the earth, rather than ramming or pounding the earthforming the wall of the cavity, smearing and remolding is minimized, thein situ soil strength charatceristic is in no way diminished, and thesoil retains substantially the degree of permeability present beforecutting.

While it is not critical that the cutting unit employed be constructedso that withdrawing it from the earth also removes the section of earthcut thereby, it is convenient and preferable to use such a unit. Forexample, an augertype drill having continual flights has provensatisfactory; the flights on the auger defining the circumference of thecavity.

Additionally, it is desirable that the cutting unit have a hollow shaftthrough which the tubing of the wellpoint may be passed. In this way,tubing may be passed 3 through the shaft, while at the same timewithdrawing the Vcutting unit, thereby providing continuous support forthe walls of the cavity. It is noted that various field test devicessuch as piezometers may be installed through the hollow shaft prior tobackflling.

If a hollow shaft cutting unit is employed, the bottom of the shaft maybe equipped with a plug or plate so as to prevent the soil penetrated bythe cutting unit from travelling up the hollow shaft. Such plug orplate, however, should be so constructed as to allow admission of theporous material into the cavity.

An `apparatus suitable for forming the cavity and backfilling same isdescribed in U.S. application Serial No. 246,411, filed December 21,1962. In clay or clay-like deposits, there are frequently alternatezones of permeable and impermeable strata, called varves When this isthe case, the formation water often finds its way to the sand drainthrough the permeable varves which are often sandy or silty. The lesspermeable varves usually consist of clay or clay-like material, throughwhich the water or other formation liquid cannot easily penetrate. Ifthe earth is removed by pulling the cutting unit, there is likelihoodthat substantial smearing of the permeable strata with the lesspermeable material will occur. It is therefore desirable to effect atleast one complete revolution of the cutting unit while holding the unitat substantially the same vertical position, to insure against smear.For exfample, when a continual fiight auger is used as the cutting unit,the auger is held in position so that it does not penetrate any deeperinto the formation; then the auger may be rotated several times. Thisgently cuts the soil away from the wall; the auger flight acting as atrimming blade. This results in the cutting away of a core of earth,forming a hole in the soil formation, wherein none of the outer wallsurface of the core is bonded with the remaining formation, so that,when the auger is removed, smearing and remolding will be minimized.

A convenient method for holding the cutting unit at substantially thesame vertical position during the operation is to provide a gearmechanism at the top of the auger, which mechanism may be so marked thatthe operator will know when he has turned the auger a completerevolution or fraction thereof.

To guard against distrubance to the subsoil, it is desirable that theauger flights advance in a controlled manner until such time as thedesired depth is reached. This may be accomplished by applying a toothat the lower portion of the mechanism supporting the auger. As the augerrotates, the flights will screw themselves down on the tooth so that theauger will advance downward uniformly. The tooth may be designed toswivel back prior to the cutting operation so that the soil core may betrimmed and the entire auger removed without interference. In puttingdown a subsequent drain hole, the tooth may be swiveled back intoposition thereby removing any soil sticking in the auger iiights as theights screw down on the tooth. Thus, the mechanism would be selfcleaning.

Oftentimes when an area has been dewatered, it is desirable to limit theradius of influence of a wellpoint system, for example, where adjacentsoils will compress when water is removed. To avoid compression andconsequent settling damage to structures in the area, the soil isrecharged to elevate the water table to its original level beyond thearea where dewatering is desired. To accomplish recharging a wellpointmay be installed according to the method described herein and waterpumped from the surface through the tubing of the wellpoint and into theformation. This technique of installing the wellpoint has the advantageof reducing soil disurbance without adversely affecting the permeabilityof the adjacent soil to any substantial degree. When the water level inthe cavity is higher than the surrounding water table, there is apressure differential which forces additional water into the soil toaccomplish rewatering.

In a broad sense, the technique for helically cutting into the soil maybe used to advantage where ever percolation into the clay or siltformation is desired. For example, the effluent from a septic tank maybe conducted into one or more cavities containing a porous material. Byhelically cutting the clay or silt to form the cavities, the naturalpermeability of the soil is not adversely affected and maximumpercolation will occur. Other uses embracing the idea of creating aplurality of cavities in a clay or slit formation and allowing theformation water to drain into the cavity or passing into theformationfrom said cavity will be apparent to those skilled in the art.

Particular aspects of my Iinvention maybe seen with reference to FIGURE1 which shows a sectional view of different kinds of cavities, andFIGURE 2 which shows sand and -a wellpoint being injected into a cavitythrough a hollow-shafted auger.

Referring to FIGURE 1, A, B, C, D, and E refer to various congurationsinvolving formation of the soil cavity and the disposition of permeablemedia and equipment within the cavity. It is noted that the invention isnot limited to the conditions therein presented to illustrate the methodreferred to in the invention.

The subsoil 1 may be stratified heterogeneously or homogeneously, orneed not be stratified. The permeability of the subsoil in its naturalstate is greater than its permeability when remolded or otherwisedisturbed as is generally the case in silty and clayey soils.

The cavity is formed by Ithe insertion of an auger into the soil in acontrolled manner so as to minimize remolding and rel-ated disturbanceto the periphery 4 of the cavity, FIG. 1A. The core of soil containedwithin the flights 3 on shaft 2 of the auger may be fully or partiallyremoved from the subsoil depending upon the specific configurationdesired and the equipment employed. If it is desired to place medium 5of specific permeability or other characteristic into the lower or anyspecific portion of the cavity, as in FIG. 1B and 1C, this may beaccomplished and the remainder of the cavity may be back-filled with anydesired material 6. Although instances occur where the cavity may befound to be self supporting, therefore requiring no backfill or cavitysupport medium, such is not generally the case. As collapse of thecavity will result in undesirable soil movements, resulting in a loss insoil permeability, cavity support may not always be indispensable butshould be provided.

The usual means for dewatering or recharging is apipe -7 to which isconnected a well-point 8 such as described in U.S. Patent 1,570,697,FIGS. 1B and E. However, depending upon the rate of water flow desiredand the character of the support mediums into which it is inserted, asimple pipe 7 may suflice and function efficiently, FIGS. 1C and 1D. Thebest means for given conditions will be evident to those familiar in theart.

The natural water table is shown as X-X, and for convenience is shown tobe above the bottom of the cavity. Where the objective is dewateringrequiring the lowering of the water table X-X, the cavity wouldnecessarily penetrate the level of the water table; whereas forrecharging purposes the cavity need not necessarily be carried throughthe level of the water table X-X, although it may, at times, bedesirable to do so. Where medium 6 is not permeable, the well-point 8 orthe low point of the pipe 7 must be below medium 6 and situated withinthe range of the permeable medium 5. For dewatering purposes, a suctionis effected by means of a suitable pump attached to pipe 7 to cause thewater or other fluid in the soil to flow from the soil 1, through thecavity periphery 4, through the porous medium 5 (if used), into the pipe7 and well-point 8 (if used) and out of the system.

By lowering the pressure sufficiently to effect a lowering of the watertable, the rate at which the water is extracted from the soil mustexceed the rate at which the water table will be replenished. Thesuction causes a lowering of the water table in the cavity to Z-Z, andbecause the water or fluid in the soil will flow into the cavity therewill be tapering of the water table in the surrounding soil for adistance beyond the cavity. The higher the permeability of the cavityperiphery, the less the suction that will be required to produce a givenlevel beyond the cavity; or for a given suction the greater thedewatering effect. By putting the fluid in the pipe 7 under pressure thereverse action occurs, raising the effective pressure in the cavity toY-Y with a flow into the soil occurring and tapering for a distancebeyond the cavity. Where the desired level of Y-Y is to be higher thanthe existing ground surface, then an impermeable plug may be employed inthe place of medium 6 in FIGS. 1B and C able to withstand the fluidpressure applied through the pipe 7 without being forced out of theground. Other plugs, caps, and seals may be employed as will be evidentto those fami-liar in the art. As in the case of dewatering, the higherthe cavity periphery permeability, the more efllcient the system. It isnoted that w represents the fluid taken from the soil or placed into thesoil.

Where only recharging is desired, and the required level of water toproduce the desired flow is lower than the natural ground surface, thepermeable medium 5 may extend to the ground surface and the fluid w maybe applied to the top of the porous medium 5 at a suitable rate througha suitable pipe or orifice 9. If this type of system is desired, but theWater level required is above the ground level, a suitable container maybe provided to retain the water at a higher Ilevel than the groundsurface. Implementation and variations of the foregoing will be evidentto those familiar in the art.

Where it is desired to measure the level of fluid in the soil, the pipe7 may be a passive pipe such that the level of fluid within the pipewill equalize with that in the soil or otherwise reflect the iluidpressure in the soil. A suitable rod, gage or other compatible systemmay then be inserted into the pipe and the water level in the systemascertained. This is useful in specific instances such as when porepressure of fluid in the soil is related to the effective stresses inthe soil which may be important in the control of the safety of specifictypes of dams. Other uses will be evident to those familiar in the art.By effecting the highest peripheral permeability in the cavityperiphery, the response time for the fluid to have the desiredinformation is reduced and greater accuracy of the reading is obtained.

I claim:

1. A method for locally controlling the fluid level in clayey or siltysoils comprising (l) forming a cavity in the soil by (a) penetratinginto the soil to the desired depth by helically cutting with a flightauger into the earth formation thereby to form a core of earth,

(b) removing the core from the soil by withdrawing the auger thereby toform a cavity,

(c) disposing a well-point at the desired depth in the cavity,

(2) actuating the well-point as a conduit for fluid flow,

(3) controlling the fluid flow in the well-point to control locally thefluid level in the soil.

2. The method of claim 1, wherein after penetrating into the formationto the desired depth, a cylindrical cut is circumscribed throughout thedepth of the penetration by making at least one complete revolution ofthe auger While maintaining the auger in the same Vertical position asthat occupied at the termination of its penetration into the earth.

3. The method of claim 1, wherein the auger is hollow shafted andwherein the cavity is filled with porous material through said hollowshaft.

4. The method of claim 1 wherein the fluid level is controlled byintroducing fluid into the soil and said wellpoint is a conduitextending to a desired depth.

5. The method of claim 4, wherein the eilluent from a septic tank is thefluid that is introduced into the soil.

6. The method of claim 1, wherein the fluid level is controlled byremoving fluid from the soil.

References Cited by the Examiner UNITED STATES PATENTS 193,348 7/1877Robinson 175-394 1,570,697 6/1926 Moore. 2,729,067 6/1956 Patterson6l-53.58 2,920,455 6/1960v Ryser et al 61-63 X 3,096,622 7/1963 Landau61-10 FOREIGN PATENTS 652,570 4/ 1951 Great Britain.

OTHER REFERENCES Construction Methods and Equipment, pub. April 1956,pp. 74, 75 and 76.

Engineering News-Record (pub), Apr. 6, 1944, pp. 81 to 85.

CHARLES E. OCOINNIELL Primary Examiner.

JACOB SHAPI'RO, EARL J. WITMER, Examiners.

1. A METHOD FOR LOCALLY CONTROLLING THE FLUID LEVEL IN CLAYEY OF SILTYSOILS COMPRISING (1) FORMING A CAVITY IN THE SOIL BY (A) PENETRATINGINTO THE SOIL TO THE DESIRED DEPTH BY HELICALLY CUTTING WITH A FLIGHTAUGER INTO THE EARTH FORMATION THEREBY TO FORM A CORE OF EARTH, (B)REMOVING THE CORE FROM THE SOIL BY WITHDRAWING THE AUGER THEREBY TO FORMA CAVITY, (C) DISPOSING A WELL-POINT AT THE DESIRED DEPTH IN THE CAVITY,(2) ACTUATING THE WELL-POINT AS A CONDUIT FOR FLUID FLOW, (3)CONTROLLING THE FLUID FLOW IN THE WELL-POINT TO CONTROL LOCALLY THEFLUID LEVEL IN THE SOIL.